I never have and I never will. First off, it's too risky because you're more likely to get scammed, both your money and your account. Second, some games punish players severely for sharing accounts. And finally, there's no reward factor, if you let someone else train your character.

I haven't paid anyone to train my character but I've done small tasks for friends if they asked me. On a game I used to play there were surveys/offers you can do to earn their micro currency for free. I did this one time for a friend cause his parents didn't allow him to do them.